The Early Legion (4th century BC)

In abandoning the phalanx, the Romans showed their genius for adaptability.
Though much of the credit might not be due to the Romans alone. For Rome was a founding member of the Latin League, an alliance initially formed against the Etruscans. The development of the early legion therefore might well be seen as a Latin development.
There were now three lines of soldiers, the hastati in the front, the principes forming the second row, and the triariirorarii and accensi in the rear.
At the front stood the hastati, who were most likely the spearmen of the second class in the previous organization of the phalanx. The hastaticontained the young fighters and carried body armour and a rectangular shield, the scutum, which should remain the distinctive equipment of the legionary throughout Roman history. As weapons they carried a sword each and javalins. Though attached to the hastati were far more lightly armed skirmishers (leves), carrying a spear and several javelins.
The soldiers of the old first class now appear to have become two types of units, the principes in the second line and the triarii in the third line. Together they formed the heavy infantry.
The principes were the picked men of experience and maturity. They were similarly, though better equipped than the hastati. In fact the principeswere the best equipped men in the early legion.
The triarii were veterans and still much looked and functioned like the heavily armed hoplites of the old Greek phalanx.
The other new units, the rorariiaccensi (and leves) represented what once had been the third, fourth and fifth class in the old phalanx system.
The rorarii were younger, inexperienced men, and the accensi were the least dependable fighters.

At the front the hastati and principes each formed a maniple of about 60 men, with 20 leves attached to each maniple of hastati.
At the back the triarii rorarii and accensi were organized into a group of three maniples, about 180 men, called an ordo.
As the historian Livy quotes the main fighting force, the principes and the hastati, at a strength of fifteen maniples then the following size could be assumed for a legion:

15 groups of leves (attached to the hastati) 300
15 hastati maniples 900
15 principes maniples 900
45 maniples (15 ordi) triarii, rorarii, accensi 2700
Total fighting force (without horsemen) 4800

The tactics were thus;
The hastati would engage the enemy. If things got too hot, they could fall back through the lines of the heavy infantry principes and re-emerge for counter attacks.
Behind the principes knelt a few yards back, the triarii who, if the heavy infantry was pushed back, would charge forward with their spears, shocking the enemy with suddenly emerging new troops and enabling the principes to regroup. The triarii were generally understood as the last defence, behind which the hastati and principes could retire, if the battle was lost. Behind the closed ranks of the triarii the army would then try to withdraw.
There was a Roman saying ‘It has come to the triarii.’ which described a desperate situation.

The famed Fluvius Camillus made some significant changes to the armament of the legion according to traditional Roman view. As the bronze helmets proved to be inadequate protection against the long swords of the barbarians, the Romans credited him with the issue of helmets made of iron with a polished surface to cause the swords to be deflected. (Though bronze helmets were later re-introduced.)
Also the introduction of the scutum, the large rectangular shield was attributable to Camillus, the Romans thought. Though in fact, in is doubtful for both the helmet as well as the rectangular scutum to have been introduced by Camillus alone.

In the early third century BC the Roman legion proved a worthy adversary against King Pyrrhus of Epirus and his well-trained Macedonian phalanx and war elephants.
Pyrrhus was a briliant tactician in the tradition of Alexander and his troops were of good quality.
The Roman legions might have been defeated by Pyrrhus (and only survived due to a near endless resource of fresh troops) but the experience gathered by fighting such an able foe was to prove invaluable for the great contests that lay ahead.
In the same century the first war against Carthage steeled the Roman army yet further, and towards the end of the century the legions defeated a new attempt by the Gauls to launch themselves southward from the Po valley, proving that now the Romans were indeed a match for the Gallic barbarians who had once sacked their capital.

At the outset of the Second Punic War, the historian Polybius tells us in his formula togatorum, Rome possessed the largest and finest army of the Mediterranean. Six legions made up of 32’000 men and 1600 cavalry, together with 30’000 allied infantry and 2’000 allied cavalry. And this was merely the standing army. If Rome called on all her Italian allies she had another 340’000 infantry and 37’000 cavalry.